Multi-user interference resilient ultra wideband (UWB) communication

ABSTRACT

Techniques are described for maintaining the orthogonality of waveforms transmitted in ultra wideband (UWB) multi-user wireless communication systems. The multi-stage block-spreading (MS-BS) techniques described herein deterministically eliminate multiple user interference (MUI) in the presence of frequency-selective fading channels. A transmitter includes a block-spreading unit to generate a stream of frames from a block of information bearing symbols by applying an orthogonal set of spreading codes, such as direct sequence code-division multiple access (CDMA) codes or digital carrier frequency multiple access codes, such that the frames corresponding to different blocks of the symbols are interleaved. The transmitter further includes a time-hopping spreading unit to generate a stream of chips from the stream of frames by applying an orthogonal set of time-hopping (TH) spreading codes such that chips corresponding to different frames are interleaved. The stream of chips may be padded with a number of guard chips determined as a function of the length of the communication channel.

[0001] This application claims priority from U.S. ProvisionalApplication Ser. No. 60/453,809, filed Mar. 8, 2003, the entire contentof which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under Subcontract#497420 awarded by the University of Delaware (Army Prime#DAAD19-01-2-011). The Government may have certain rights in theinvention.

TECHNICAL FIELD

[0003] The invention relates to communication systems and, moreparticularly, transmitters and receivers for use in multi-user ultrawideband (UWB) communication systems.

BACKGROUND

[0004] In multi-user wireless communication systems, such as wirelesslocal area networks, satellite communications and mobile phone networks,multiple transmitters and receivers may communicate simultaneouslythrough a common wireless communication medium. One communication formatwith attractive features for high data rates and low power consumptionis ultra wideband (UWB), also known as impulse radio (IR), in which thetransmitter generates a train of ultra short pulses spreading the energyof the transmitted signal across an ultra wide bandwidth. IR technologyemploying time-hopping (TH) codes pseudo randomly spreads pulses intime, and is often referred to IR multiple access (IRMA).

[0005] In general, IRMA applies a TH code chosen from a set oforthogonal “spreading codes” to an outbound stream of “symbols.” EachIRMA pulse transmits a “symbol” representing a discrete informationbearing value selected from a finite set (“alphabet”). For example,simple alphabets used by transmitters may be {+1, −1} or {−3, −1, +1,+3}. TH codes apply a finite set of integer values to time shift asequence of transmitted pulses and produce a set of “chips” for eachvalue to be transmitted. Each TH code has a sequence, resulting in eachIRMA pulse train having a defined time period. The resulting chips aretransmitted according to some modulation scheme, such as pulse positionmodulation (PPM). In order to separate signals from multiple users, thereceivers isolate the signal of the desired user by matching the user'ssignal to the corresponding orthogonal spreading code associated withthat user.

[0006] IRMA systems often operate in dense multi-path environments. IRsignaling provides resolvable multi-path components and enablescollecting energy from multi-path propagation with appropriate receiverdesign. However, when multiple users are present, the multi-pathpropagation also induces multi-user interference (MUI). In addition, thecommunication channel between the transmitter and the receiver can alsobecome “frequency selective” in that certain frequencies exhibit fading,i.e. significant loss of signal. Consequently, inter-symbol interference(ISI) in which the transmitted symbols interfere with each otherdestroys the orthogonality of the waveforms at the receiver. MUI,together with ISI, can cause the receiver to be unable to correctlyseparate the multi-user waveforms, eventually leading to data lossand/or bandwidth and power inefficiencies.

[0007] Various “multi-user detectors” have been developed for separatingnon-orthogonal UWB multi-user waveforms. These multi-user detectors,however, are often complex and expensive to implement in typical mobilecommunication devices, and typically require knowledge of thecharacteristics of the current communication channel. For example,linear detectors, such as decorrelating and minimum mean-squared error(MMSE) detectors, often require inversion of large matrices with sizeincreasing in proportion to the square of the number of users, whileoptimum maximum likelihood (ML) detectors entail exponential complexity.In addition, some analog IRMA systems approximate MUI as Gaussian noiseand attempt to suppress it statistically. Such systems requiresuccessful application of strict power control and rely on the Gaussianapproximation. However, when the number of users is not sufficientlylarge, the Gaussian approximation is not valid.

SUMMARY

[0008] In general, techniques are described for deterministicallysubstantially eliminating multi-user interference (MUI) in multi-userwireless communication systems, such as ultra wideband (UWB) systemsUnlike conventional IRMA systems in which MUI is approximated asGaussian noise and rely on time-hopping (TH) spreading codes tostatistically suppress MUI, “multi-stage block-spreading” (MS-BS)techniques are described that eliminate MUI deterministically at thereceiver by preserving the orthogonality between different userswaveforms through multi-path channels. Furthermore, the techniquesdescribed herein allow a larger number of users than conventional IRMAsystems and can provide different users with variable transmissionrates.

[0009] In one embodiment, the invention is directed to a method whichgenerates a stream of frames from blocks of information bearing symbolswherein the frames corresponding to different blocks of the symbols areinterleaved and generates a stream of chips from the stream of frames,wherein the chips corresponding to different frames are interleaved. Anultra wideband (UWB) transmission signal is output from the stream ofchips. The stream of frames is generated by parsing the symbols intoblocks of K symbols, applying an orthogonal set of spreading codes tothe blocks of K symbols to form Q frames, and interleaving the Q framesto form the stream of frames. The stream of chips is generated byapplying an orthogonal set of time-hopping spreading codes to theinterleaved frames to generate a plurality of chips for each frame, andinterleaving each of the plurality of chips to form the output stream ofchips.

[0010] The method may further assign each of the set of spreading codesto a different user of a group of users and assign each user of thegroup a common one of the time-hopping spreading codes. Unique addressesare assigned to users as unique pair-wise combinations of the set oforthogonal spreading codes and the set of time-hopping spreading codes.

[0011] In additional embodiments, the method receives the output signaland outputs a stream of estimate symbols from the signal using atwo-stage de-spreading unit having a time-hopping de-spreading moduleand a multi-user de-spreading module.

[0012] In another embodiment, the invention is directed to a wirelesscommunication device having a multiple-user block-spreading unit thatgenerates a set of frames for respective blocks of information bearingsymbols and produces a stream of frames in which the frames fromdifferent sets are interleaved, a time-hopping block-spreading unit thatgenerates a set of chips for each frame and outputs a stream of chips inwhich the chips generated from different frames are interleaved, and apulse shaping unit to output an UWB transmission signal from the streamof interleaved chips.

[0013] In yet another embodiment, the invention is directed to awireless communication device having a two-stage de-spreading unit thatprocesses a received ultra wideband (UWB) transmission signal to produceestimate symbols, wherein the received UWB signal comprises a multi-userblock-spread UWB signal formed from interleaved symbol frames andinterleaved chips within the symbol frames.

[0014] In additional embodiments, the invention is directed to a systemhaving a wireless transmitter to transmit an ultra wideband (UWB) signalaccording to interleaved chips generated from interleaved framesproduced by blocks of information bearing symbols and wireless receiverto receive the UWB signal and de-interleave the chips and frames toproduce estimate symbols.

[0015] In another embodiment, the invention is directed to acomputer-readable medium containing instructions. The instructions causea programmable processor of a wireless communication device to generatea stream of frames from blocks of information bearing symbols, whereinthe frames corresponding to different blocks of the symbols areinterleaved, generate a stream of chips from the stream of frames,wherein the chips corresponding to different frames are interleaved, andoutput an UWB transmission signal from the stream of chips.

[0016] The multi-stage block-spreading techniques described herein mayoffer one or more advantages. For example, MS-BS preserves theorthogonality of a transmitted UWB signal through a multi-usercommunication channel. Consequently, transmitted signals are resistantto MUI regardless of the underlying frequency selective nature of themulti-path communication channel without using adaptive power control todynamically adjust the usage of power by transmitter. As a result, thetechniques render the multiple access communication channel equivalentto a set of independent parallel single-user frequency-selectivechannels with additive white Gaussian noise (AWGN). In other words,because the transmitted signals remain orthogonal, IRMA receivers may beimplemented with single-user detectors, which are less complex thanmulti-user detectors. Furthermore, the receiver can be implemented infavor of low complexity or high performance.

[0017] Other advantages of the described techniques may include improvedbandwidth efficiency, which implies that information can be transmittedat higher rates and that the maximum allowable number of simultaneoususers increases. Furthermore, as the number of users increases, an IRMAsystem that incorporates the described techniques may be more resilientto degradation in bit-error-rate (BER) performance than conventionalIRMA systems.

[0018] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0019]FIG. 1 is a timing diagram illustrating an example time-hopping(TH) pulse position modulation (PPM) scheme in an impulse radio multipleaccess (IRMA) system.

[0020]FIG. 2 is a block diagram illustrating a wireless system impulseradio multiple access (IRMA) format in which multiple transmitterscommunicate with multiple receivers through a channel.

[0021]FIG. 3 is a block diagram illustrating in further detail themulti-user UWB communication system of FIG. 2.

[0022]FIG. 4 illustrates an example assignment of multiple users (MU)and time-hopping (TH) addresses.

[0023]FIG. 5 is a block diagram illustrating an example transmission ofa symbol using PPM in an IRMA system.

[0024]FIG. 6 illustrates an example data stream generated by amulti-user block-spreading unit within a UWB transmitter of FIG. 3.

[0025]FIG. 7 illustrates an example data stream generated by a THblock-spreading unit within the transmitter of FIG. 3.

[0026]FIG. 8 is a flowchart illustrating an example mode of operation ofa transmitter in the UWB communication system of FIG. 3.

[0027]FIG. 9 is a flowchart illustrating an example mode of operation ofa receiver in the UWB communication system of FIG. 3.

[0028]FIGS. 10-13 are graphs illustrating modeled performance estimatesof the techniques described herein.

DETAILED DESCRIPTION

[0029] The techniques described herein use notation defined as follows.Bold upper case letters and lower case letters denote matrices andcolumn vectors respectively. ( )^(T) and ( )^(R) denote transpose andHermitian transpose respectively. δ( ) and

stand for Kroenecker's delta and Kronecker's product respectively. E { }denotes expectation. └ ┘ and ┌ ┐ denote integer floor and integerceiling respectively. I_(K) denotes the identity matrix of size K andO_(M×N) denotes an all zero matrix of size M×N.

[0030]FIG. 1 is a timing diagram that illustrates an exampletransmission of two separate symbols, 1(1A) and 0 (1B), using binarypulse position modulation (PPM) in an impulse radio multiple access(IRMA) format. Generally, the PPM-IRMA format described herein isdesigned such that the PPM delays are sufficiently large to ensureorthogonal modulation using M-ary PPM. As a result, a nonlinear PPM-IRMAformat can be viewed as a linearly modulated code-division multipleaccess (CDMA) format.

[0031] Each symbol 1A, 1B represents a discrete information bearingvalue selected from a finite set (“alphabet”), e.g. {1, 0}. Symbols 1A,1B are repeated over N_(f) frames, e.g. N_(f)=5, with each frame 2having duration T_(f) 4. During signaling interval T_(s) 6 of durationT, =N_(f)T_(f) seconds, k_(b)=logg₂ M message bits having bit rate R_(b)may be loaded into a k_(b)-bit buffer. The resulting k_(b)-bit outputsymbol rate is R_(s)=R_(b)/k_(b). For example, the u^(th) user'sinformation bearing symbol transmitted during the k^(th) frame isdenoted I_(u)(└k/N_(f)┘) where I_(u)(└k/N_(f)┘)∈[M−1].

[0032] The application of a time-hopping (TH) spreading code to theinformation bearing symbols produces a set of “chips” over which eachsymbol 1A, 1B is transmitted. Each frame 12 comprises N., e.g. N_(c)=3,chips with each chip 8 having chip duration T_(c) 9. Frame durationT_(f) is then equivalent to N_(c) T_(c)+T_(g), where T_(g) is a guardtime to account for processing delay at the receiver between twosuccessively received frames. For simplicity T_(g) is set to zero forall following equations. A nonzero value for T_(g) does not impose anylimiting consequences. The u^(th) user's transmitted waveform v_(u)(t)18 is given in accordance with equation (1): $\begin{matrix}{{v_{u}(t)} = {P_{u}{\sum\limits_{- \infty}^{+ \infty}{w\left( {t - {kT}_{f} - {{c(k)}T_{c}} - \tau_{I_{u}{({\lfloor{k/N_{f}}\rfloor})}}} \right)}}}} & (1)\end{matrix}$

[0033] where P_(u) is the u^(th) user's transmission power, w(t) denotesthe ultra-short pulse 7 and {dot over (c)}_(u)(k)∈[0, N_(c)−1] 11 is aperiodic pseudo random sequence with period PC equal to N_(f).Ultra-short pulse 7 typically has a duration between 0.20 and 2.0nanoseconds and may be selected as a Gaussian monocycle, a Gaussianbiphase monocycle, a doublet consisting of a positive Gaussian pulsefollowed by its negative, and the like. The role of {dot over(c)}_(u)(k) 3 is to enable both multiple users over a communicationchannel and security.

[0034] Each signaling interval T_(s) 6 of transmitted waveform 18 inequation (1) includes N_(f) copies of a single symbol 1A, 1B, i.e. oneper frame 2, with pulse 7 time-shifted in each frame 2 according to thesymbol value, e.g. it is shifted by τ_(m) for I_(u)(└k/N_(f)┘)=m where m∈[0, 1, . . . , M−1]. In order to ensure orthogonal modulation, PPMmodulation delays τ_(m) must satisfy τ_(m)−τ_(m−1)≧T_(w) for ∀ m ∈[1,2,. . . , M−1]. Thus, chip duration T_(c) 9 should be chosen to satisfyT_(c)≧τ_(m−1)+T_(w)≧MT_(w).

[0035] The orthogonal M-ary PPM of the u^(th) user can be viewed ashaving M parallel branches with each parallel branch realizing a shiftedversion of the pulse stream. Consequently, in order to generate thewaveform v_(u)(t) as given in equation (1), only one branch out of the Mparallel branches needs to be selected depending on the symbol value.Summing the M parallel branches, waveform v_(u)(t) can be rewritten interms of waveform v_(u,m)(t) as given in equations (2) and (3)respectively. $\begin{matrix}{{v_{u}(t)} = {\sum\limits_{m = 0}^{m = {M - 1}}{v_{u,m}(t)}}} & (2) \\{{v_{u,m}(t)} = {P_{u}{\sum\limits_{k = {- \infty}}^{k = {+ \infty}}{{s_{u,m}\left( \left\lfloor {k/N_{f}} \right\rfloor \right)}{w\left( {t - {kT}_{f} - {{c(k)}T_{c}} - \tau_{m}} \right)}}}}} & (3)\end{matrix}$

[0036] The waveform s_(u,m)(└k/N_(f)┘):=δ(I_(u)(└k/N_(f)┘)−m), ∀ m ∈[0,M−1] (12). Equation (3) can be rewritten as equation (4) by definingmonocycles 7 as pulse functions w_(m)(t):=w(t−τ_(m)) and using thesubstitution T_(f)=N_(c)T_(c). $\begin{matrix}{{v_{u,m}(t)} = {P_{u}{\sum\limits_{k = {- \infty}}^{k = {+ \infty}}{{s_{u,m}\left( \left\lfloor {k/N_{f}} \right\rfloor \right)}{w_{m}\left( {t - {\left( {{kN}_{c} + {c(k)}} \right)T_{c}}} \right)}}}}} & (4)\end{matrix}$

[0037] Because {dot over (c)}(k)3∈[0, N_(c)−1] is an integer, w_(m)(t)is shifted by an integer multiple of T_(c) 9. As a result waveformv_(u,m)(t) can be written in equation (5) accordingly. Equation (5) canbe viewed as a linearly modulated waveform with symbol rateR_(s)=1/T_(c) while equation (2) can be viewed as the superposition of Mlinear modulators, each with a different pulse function w_(m)(t).$\begin{matrix}{{v_{u,m}(t)} = {P_{u}{\sum\limits_{k = {- \infty}}^{k = {+ \infty}}{{v_{u,m}(n)}{w_{m}\left( {t - {nT}_{c}} \right)}}}}} & (5)\end{matrix}$

[0038] Sequence v_(u,m)(n) is dependent on s_(u,m)(k) and c_(u)(k) 3.Chip-rate code sequence c_(u)(n) 5 with periodP_(c)=N_(c)P_(c{dot over (c)}) is defined via {dot over (c)}(k) 11 asgiven in equation (6).

c _(u)(n):=δ(└n/N _(c) ┘N _(c) +c _(u)(└n/N _(c)┘)−n)  (6)

[0039] The relation between {dot over (c)}_(u)(k) 3 and c_(u)(n) 5 isgiven by the mapping between chip index n and frame index k. As aresult, the u^(th) user's chip sequence on the m^(th) branch v_(u,m)(n)can be expressed according to equation (7).

v _(u,m)(n)=s _(u,m)(└n/N _(c) N _(f)┘)c _(u)(n)  (7)

[0040] Sequence s_(u,m)(└n/(N_(c)N_(f))┘) does not change over theduration of N_(f)N_(c) chips (N_(f)T_(f) seconds) and is spread byc_(u)(n) 5 in equation (2) to generate chip sequence v_(u,m)(n) inequation (7). From this chip-rate sampled model, the nonlinearlymodulated PPM-IRMA format can be viewed as a linearly modulatedcode-division multiple access (CDMA) format.

[0041] Although T_(g) was set equal, the model can also include T_(g) asnonzero. This can be accomplished by setting T_(g)=N_(g)T_(c) with N_(g)being an integer and restricting {dot over (c)}c(k) 3 to take on values[0, N_(c)′−1], where N_(c)′:=N_(c)−N_(g). Hereafter, all equations andderivations will assume T_(g)=0.

[0042]FIG. 2 is a block diagram illustrating multi-user wirelesscommunication system 10 in which multiple transmitters 12 communicatewith receivers 14 through wireless channel 16 via the M-ary PPM-IRMAformat described in FIG. 1. In general, the invention providestechniques for deterministically substantially eliminating multi-userinterference (MUI) 18 in communication system 10 that could beintroduced in waveforms produced by transmitters 12 during transmissionthrough communication channel 16.

[0043] Transmitters 12 transmit information using techniques referred toherein as “multi-stage block-spreading” (MS-BS). MS-BS generates astream of frames from blocks of information symbols wherein the framescorresponding to different blocks of the symbols are interleaved and astream of chips is generated from the stream of frames wherein the chipscorresponding to different frames are interleaved. Transmitters 12output a UWB transmission signal from the stream of chips.

[0044] The stream of frames is generated by applying an orthogonal setof spreading codes, such as direct sequence CDMA codes, TDMA codes, orFDMA codes, while the stream of chips is generated by applying anorthogonal set of TH spreading codes. The stream of chips may be paddedwith a number of guard chips determined as a function of length ofchannel 16. The set of spreading codes and the set of TH spreading codesare mutually exclusive so that the interleaved and padded chips retaintheir orthogonality after passing through channel 16. Each user of agroup of users is assigned a spreading code from the set of spreadingcodes and is also assigned a common TH spreading code from the set of THspreading codes. System 10 supports a total number of users determinedby the product of the number of frames over which an information symbolis repeated N_(f) and the number of chips in each frame N_(c).

[0045] MS-BS techniques preserve the orthogonality between differentusers signals through communication channel 16 regardless of theunderlying frequency selective nature of the multi-path communicationchannel thereby rendering channel 16 equivalent to a set of independentparallel single-user frequency-selective channels with additive whiteGaussian noise (AWGN).

[0046] Receivers 14 receive the UWB transmission signal output bytransmitters 12 and output a stream of estimate symbols from thereceived signal using a two-stage de-spreading unit having a THde-spreading module and a multi-user de-spreading module. The stream ofoutput symbols is generated by converting the received signal to astream of chips and applying a first de-spreading matrix with the THde-spreading module to de-interleave the chips into blocks of frames.The multi-user de-spreading module applies a second de-spreading matrixto the blocks of frames to de-interleave the frames and produce blocksof estimate symbols. A single user detection scheme is applied to theblocks of estimates symbols to output the stream of the estimatesymbols.

[0047] These techniques work with existing communication systems usingIRMA with orthogonal M-ary pulse position modulation (PPM) to eliminateMUI 18 deterministically at receivers 14 and apply to uplink anddownlink transmissions, i.e., transmissions from a base station to amobile device and vice versa. Transmitters 12 and receivers 14 may beany device configured to communicate using a multi-user wirelesstransmission including a hub for a wireless local area network, acellular phone, a laptop, handheld computing device, a personal digitalassistant (PDA), or a cellular distribution station, and the like.

[0048] The techniques described herein may be applied to uplink and/ordownlink transmissions, i.e., transmissions from a base station to amobile device and vice versa. Consequently, transmitter 12 and receiver14 may be any device configured to communicate using a multi-user ultrawideband wireless transmission including a distribution station, a hubfor a wireless local area network, a mobile phone, a laptop or handheldcomputing device, a personal digital assistant (PDA), a device within awireless personal area network, a device within a sensor network, orother device.

[0049]FIG. 3 is a block diagram illustrating in further detailmulti-user communication system 10 of FIG. 2 using multi-stageblock-spreading (MS-BS). For simplicity, the m_(th) branch of atransmitter 12 and the m′^(th) branch at a receiver 14 are given indetail for the u^(th) user. Transmitter 12 applies MS-BS techniques andpulse shaping to chip-rate information bearing symbols s_(u)(m,n) 12 andtransmits the MS-BS M-ary pulse position modulated (PPM) IRMA waveformv_(u,m)(t) 28. Receiver 14 receives chip-rate sampled sequence x_(m′)(n)38 and outputs symbol estimates 49.

[0050] Generally, multiple transmitters 12 corresponding to differentusers assign each user a unique spreading code from a set of orthogonalspreading codes, such as direct sequence CDMA codes TDMA codes, or FDMAcodes. Additionally each user is assigned a TH spreading code from a setof orthogonal spreading codes such that within a group of users, eachuser with a unique orthogonal spreading code is also assigned a commonTH spreading code. Therefore, a unique address {u_(A), U_(B)} can beassigned to each user with the first index of the address indicating theTH code assigned to the u^(th) user and the second index indicating theorthogonal spreading code assigned to the u^(th) user.

[0051] Serial to parallel (S/P) converter 20 of transmitter 12 parsesserial chip-rate data stream s_(u,m)(n) 20 into blocks of K symbols 22,each symbol representing a discrete information bearing value, asdefined in equation (8).

s _(u,m)(i):=[s_(u,m)(iK), . . . , s_(u,m)(iK+K−1)]^(T)  (8)

[0052] As described in detail below, multi-user block-spreading unit 23applies an orthogonal spreading code according to the address assignedto the u^(th) that generates a stream of Q frames from blocks of Kinformation bearing symbols and interleaves the frames corresponding todifferent blocks of the symbols. Each block of K symbols is spread intoN_(f)K×1, where N_(f)K=Q, output vectors 24 according to equation (9)via Q×K spreading matrix D_(uB) given in equation (10).

_(u,m)(i)=D _(uB) s _(u,m)(i), where

_(u,m)(i):=[{tilde over (s)} _(u,m)(iQ), . . . , {tilde over(s)}_(u,m)(iQ+Q−1 )]^(T)  (9)

D_(uB):=d_(uB)

I_(K)  (10)

[0053] In other words, the K symbol blocks are symbol-spread into Qframe-rate signals and then frame-interleaved to provide data stream 24according to equation (9). In rewriting equation (9), it can be seenmulti-user block-spreading unit 23 implements a multi-code transmitterwith K codes per user as given in equation (11). $\begin{matrix}{{{\overset{ˇ}{s}}_{u,m}(i)} = {\sum\limits_{k = 0}^{k = {K - 1}}{{s_{u,m}\left( {{iK} + k} \right)}d_{uB}^{(k)}}}} & (11)\end{matrix}$

[0054] Each column of spreading matrix D_(uB) can be viewed as aseparate spreading code for the u^(th) user, with the (k+1)^(st) columndenoted by _(duB) ^((k)).

[0055] Similarly, TH spreading unit 25 applies a TH spreading codeselected from a set of mutually orthogonal TH spreading codes accordingto the address assigned to the u^(th) user. Each block of Q frame-ratesignals of data stream 43 is spread into a stream of P chips wherein thechips corresponding to different frames are interleaved. In other words,TH block-spreading unit spreads a block of Q frame-rate signals into Pchip-rate signals followed by chip interleaving and zero padding viaspreading matrix C_(uA) according to equation (12).

v _(u,m)(i)=C _(uA)

_(u,m)(i)  (12)

[0056] TH spreading matrix C_(uA) is derived from equations (13, 14) anddefined in equation (15).

c _(uA) ^((q)) :=[c _(u.A)(qN _(c)), C _(uA)(qN _(c)+1), . . . , c_(uA)(qN _(c) +N _(c)−1)]^(T), for q∈[0, N _(f)−1]  (13)

[0057] Recall that C_(uA) is given by equation (6). Matrix C_(uA)(q) isdefined in equation (14).

C _(uA) ^((q)) =c _(uA) ^((q))

T_(zp), where T_(zp) :=[I _(K), 0_(K×L)]^(T) for  (14)

[0058] Zero-padding matrix T_(zp) is a (K+L)×K matrix and appends Lzeros at the end of each column upon multiplication. The guard chips maybe null values as implemented here with zero padding via matrixmultiplication. TH spreading matrix C_(uA) can then be written as inequation (16).

C _(uA) :=diag{C _(uA) ⁽⁰⁾ , C _(uA) ⁽¹⁾ , . . . , C _(uA) ^(N) ^(_(f))⁻¹)}  (15)

[0059] TH spreading matrix C_(uA) is of size P×Q, where P=N_(f)N_(c)(K+L). The parameter L is determined by the effective length ofchannel 16 in discrete time and is calculated below. As a result, datastream 26 at the output of TH block-spreading unit 25 is given by theP×1 vector on the m^(th) branch in equation (16) and is given inmatrix-vector form according to equation (17).

v _(u,m)(i):=[v _(u,m)(iP), . . . , v _(u,m)(iP+P−1)]^(T)  (16)

v _(u,m)(i)=C _(uA) D _(uB) s _(u,m)(i)  (17)

[0060] At the output of the two-stage MS_BS parallel to serial (P/S)converter 27 parses blocks of P chips into serial data stream v_(u,m)(n)28. MS-BS techniques may also offer different users variabletransmission rates. Assigning the u^(th) user a set of A_(u) MU and THaddress pairs denoted by ω_(u) allows equation (17) to be rewritten asequation (18). $\begin{matrix}{{v_{u,m}(i)} = {\sum\limits_{{\{{\omega_{A},\omega_{B}}\}} \in \Omega_{u}}{C_{\omega \quad A}D_{\omega \quad B}{{\overset{ˇ}{s}}_{u,m}^{\{~{\omega_{A},\omega_{B}}\}}(i)}}}} & (18)\end{matrix}$

[0061] In other words, the u^(th) user can transmit A_(u) symbol blocksof length K simultaneously, each carried on a distinct address pair{ω_(A), ω_(B)}∈ω_(u). Consequently, the u^(th) user can transmit a totalnumber of K_(u)=A_(u) K symbols during each burst of PT_(c) seconds witha corresponding transmission rate R_(u)=K_(u)/(PT_(c)). In order toavoid address collisions and guarantee deterministic symbol blockseparation, the address must be assigned such that additional conditions(19, 20) are satisfied. $\begin{matrix}{{{\bigcup\limits_{u}\Omega_{u}} \in \Omega},{\forall{u \in \left\lbrack {0,{N_{u} - 1}} \right\rbrack}}} & (19) \\{{{\Omega_{u}\bigcap\Omega_{\mu}} = \varnothing},{\forall u},{\mu \in \left\lbrack {0,{N_{u} - 1}} \right\rbrack},{{{and}\quad u} \neq \mu}} & (20)\end{matrix}$

[0062] Data stream 28 propagates through equivalent channel 30 whichincludes pulse shaper 32, physical channel 16, AWGN noise 34, andmatched filter 36. Pulse shaper 32 converts serial data stream 28 to theanalog transmitted signal 31 v_(u,m)(t) by varying the interval betweenpulses according to the data being modulated and according to theassigned TH spreading code by applying previously defined pulse shapingfunctions, w_(m)(t):=w(t−τ_(m)). Analog signal 31 propagates throughmulti-path frequency selective physical channel 16, denoted g_(u)(t) inwhich AWGN 34 is added to the signal and is filtered by matched filter36 on the m′^(th) branch, w_(m′)(t), matched to w_(m′)(t) where m′∈[0,M−1].

[0063] The chip-sampled discrete time equivalent finite impulse response(FIR) channel can be represented according to equation (21) where *denotes convolution.

h_(u,m′,m)(l):=(w _(m) *g _(u) *w _(m))(t)|_(t=lT) _(c)   (21)

[0064] Equivalent FIR channel of equation (21) of order L_(u) includesthe u^(th) user's asynchronism in the form of delay factors as well astransmit-receive filters, and the multi-path effects. AWGN 34, denotedθ(t), is effectively sampled at chip-rate, t=nT_(c), and can berepresented as sampled AWGN noise according to equation (22).

θ_(m′)(n):=(η*{overscore (w)} _(m′))(t)|_(t=nT) _(c)   (22)

[0065] Receiver 14 receives the chip-sampled matched filter output 28from matched filter 36 as given in equation (23). $\begin{matrix}{x_{m^{\prime}\quad {(n)}} = {{\sum\limits_{u = 0}^{N_{u} - 1}{\sum\limits_{m = 0}^{M - 1}{\sum\limits_{l = 0}^{L}{P_{u}{h_{u,m^{\prime},m}(l)}{v_{u,m}\left( {n - l} \right)}}}}} + {\eta_{m^{\prime}}(n)}}} & (23)\end{matrix}$

[0066] N_(u) is the number of users, L is the maximum length ofcommunication channel 16 in discrete time for the u^(th) user, and M isthe number of PPM pulse shapers.

[0067] Asynchronism among users in the uplink of a quasi-synchronoussystem, i.e. a system in which there is a coarse timing reference, islimited to a few chip intervals. The maximum asynchronism, τc_(max,a),arises between the nearest and the farthest mobile users and the maximummulti-path spread, τ_(max,s), can be found using field measurements fromthe operational environment. The maximum channel order can then bedetermined by equation (24).

L=┌(τ _(max,s)+τ_(max,a))/T _(c)┐  (24)

[0068] The downlink model, e.g. from the base station to user ofinterest μ, is subsumed by the uplink model presented above by settingh_(u,m′,l)(l)=h_(u,m′,l)(l), ∀u∈[0, N_(u)−1], since the latter allowsfor distinct user channels. Downlink transmissions are synchronous withτ_(max,a)=0 and maximum channel order L depends only on τ_(max,s)through L=┌(τ_(max,s)/T_(c)┐. The only channel knowledge assumed attransmitters 12 in the uplink or downlink model is L.

[0069] At receiver 14, serial to parallel unit 40 converts chip-ratesampled sequence 38 into P×1 blocks 41 as given in equation (25).

x _(m′)(i):=[x _(m′)(iP), x _(m′)(iP+1), . . . , x_(m′)(iP+P−1)]^(T)  (25)

[0070] Chip-rate sampled blocks 41 are received in the presence of P×1noise blocks given in equation (26).

θ_(m′)(i):=[η_(m′)(iP), η_(m′)(iP+1), . . . , η_(m′)(iP+P−1)]^(T)  (26)

[0071] Allowing H_(u,m′,m,0) to be the P×P lower triangular Toeplitzmatrix with first column [h_(u,m′,m)(0), . . . , h_(u,m′,m)(L), 0, . . ., 0]^(T) and H_(u,m′,m,1) be the P×P upper triangular Toeplitz matrixwith first row [0, . . . , 0, h_(u,m′,m)(L), . . . , h_(u,m′,m)(1)]^(T)the input-output block relationship through equivalent channel 30 can bedescribed in matrix form according to equation (27). $\begin{matrix}{{x_{m^{\prime}}(i)} = {\sum\limits_{u = 0}^{N_{u} - 1}{\sum\limits_{m = 0}^{M - 1}{P_{u}\left\lbrack {{H_{u,m^{\prime},m,0}{v_{u,m}(i)}} + {H_{u,m^{\prime},m,1}{v_{u,m}\left( {i - 1} \right)}} + {\eta_{m^{\prime}}(i)}} \right.}}}} & (27)\end{matrix}$

[0072] With v_(u,m)(i) as defined in equation (17) and making use of themathematical address assigning rules defined below, equation (27) can berewritten as given in equation (28). $\begin{matrix}{{x_{m^{\prime}}(i)} = {\sum\limits_{u_{A} = 0}^{N_{c} - 1}{\sum\limits_{u_{B} = 0}^{N_{f} - 1}{\sum\limits_{m = 0}^{M - 1}{P_{u}{\quad\left\lbrack {\quad{\quad{{H_{u,m^{\prime},m,0}C_{uA}D_{uB}{s_{u,m}(i)}} + \quad {H_{u,m^{\prime},m,1}C_{uA}D_{uB}s_{u,m}{\quad{\left( {i - 1} \right) + {\eta_{m^{\prime}}(i)}}}}}}} \right.}}}}}} & (28)\end{matrix}$

[0073] The s_(u,m)(i-1) dependent term in equation (28) accounts forinter-block interference (IBI) and will be shown to equal a 0_(P×1)matrix.

[0074] Receiver 14 performs multi-user separation on the P×1 chip ratesampled blocks 41 by applying de-spreading matrices

_(uA) and

_(uB) in TH de-spreading unit 42 and multi-user de-spreading unit 44respectively. P×N_(f)(K+L) TH de-spreading matrix

_(uA) and N_(f)(K+L)×(K+L) multi-user de-spreading matrix

_(uB) are given in equations (29, 30) respectively.

_(uA)

:=diag{

_(uA) ⁽⁰⁾

,

_(uA) ⁽¹⁾

, . . . ,

_(uA) ^(N) ^(_(f)) ⁻¹}  (29)

_(uB) :=d _(uB)

I _(K+L)  (30)

[0075] TH de-spreading matrix

_(uA) is similarly derived as C_(uA) from equations (6, 13) and equation(31).

_(uA) ^((q)) :=c _(uA) ^((q))

I _(K+L)  (31)

[0076] TH de-spreading unit 42 and MU de-spreading unit 44 performmulti-stage block-de-spreading and enable separation of the μ^(th)user's signal from superimposed multi-user signals deterministically. THde-spreading matrix

_(uA) can be viewed as chip-de-interleaving followed byblock-de-spreading. Similarly, multi-user de-spreading matrix

_(uB) can be viewed as frame-de-interlaving followed byblock-de-spreading.

[0077] Transceiver pairs {C_(uA),

_(uA)}_(uA=0) ^(N) ^(_(e)) ⁻¹ and {D_(uB)

_(uB)}_(uB=0) ^(Nf−1) are designed such that superimposed multi-usersignals can be separated deterministically regardless of multi-pathpropagation through frequency selective ISI channels of order L. Theproperties of the Kronecker product can be used to verify the followingmutual orthogonality relationships between any two users u and μ asgiven in equations (32-35).

C _(μA) ^(H) C _(uA)=δ(μ_(A) −u _(A))I _(N) _(f) _(K)  (32)

_(uA) ^(H)

_(uA)=δ(μ_(A) −u _(A))I _(N) _(f) _((K+L))  (33)

D _(μB) ^(H) D _(uB) =N _(f)δ(μ_(B) −u _(B))I _(K)  (34)

_(μB) ^(H)

_(uB) =N _(f)δ(μ_(B) −u _(B))I _(K+L)  (35)

[0078] The multiple-stage block-de-spreading performed by THde-spreading unit 42 and multi-user de-spreading unit 44 separate theμ^(th) user's signal from 41 and output MUI free block according toequation (36)

y _(μ,m′)(i)=

^(H) _(μ)

^(H) μAx _(m′)(i), ∀m′∈[0, M−1]  (36)

[0079] MUI free output 45 given by equation (37) can then be input intoany single-user detector 46 to eliminate channel effects and outputsymbol block estimates.

y _(μ)(i):=[y ^(T) _(μ,0)(i), y ^(T) _(μ,1)(i), . . . , y^(T) μ,M−1(i)]^(T)  (37)

[0080] Equations (36, 37) can be rewritten to explicitly show that thesuperimposed received signals from multiple users can be separateddeterministically regardless of the FIR multi-path channel when choosingtransceiver pairs {C_(uA),

_(uA)}_(uA=0) ^(N) ^(_(e)) ⁻¹ and {D_(uB)

_(uB)}_(uB=0) ^(Nf−1). By design, the last L rows of TH spreading matrixC_(uA) are zeros while the nonzero elements of H_(u,m′,m,1) appear onlyin its last L columns. Equations (38, 39) prove IBI interference isdeterministically eliminated.

H _(u,m′,m,1) C _(uA)=0_(PxN) _(^(f)) _(K)  (38)

H _(u,m′,m,1) C _(uA) D _(uB) S _(u,m)(i−1)=0_(P×1)  (39)

[0081] Further, equations (43, 44) derived from following equations(40-42) explicitly show that the superimposed received signals frommultiple users can be separated deterministically after propagationthrough FIR multi-path channels. This is due to the design of mutuallyorthogonal transceiver pairs {C_(uA),

_(uA)}_(uA=0) ^(N) ^(_(e)) ⁻¹ and {D_(uB)

_(uB)}_(uB=0) ^(Nf−1) given in equations (32-35) which preserves thecode orthogonality among users. In the following equations Ĥ_(u,m′,m) isa tall Toeplitz matrix of dimension (K+L)×K.

Ĥ _(u,m′,m) C _(uA) D _(uB) s _(u,m)(i)=

_(uA)

_(uB) Ĥ _(u,m′,m) s _(u,m)(i)  (40) $\begin{matrix}{{x_{m^{\prime}}(i)} = {\sum\limits_{u_{A} = 0}^{N_{c} - 1}{{\overset{ˇ}{C}}_{uA}{\sum\limits_{{uB} = 0}^{N_{f} - 1}{{\overset{ˇ}{D}}_{uB}{\sum\limits_{m = 0}^{M - 1}{P_{u}\left\lbrack {{{\hat{H}}_{u,m^{\prime},m}{s_{u,m}(i)}} + {\eta_{m^{\prime}}(i)}} \right.}}}}}}} & (41)\end{matrix}$

[0082] Equation (41) is derived by substituting equation (39) intoequation (28) and using the equality in equation (40) which showschannel matrix H_(u,m′,m,0) commutes with TH and MU spreading matricesC_(uA) and D_(uB). Using equations (32-35) and equation (41), equation(36) can be rewritten according to equation (42). $\begin{matrix}{{y_{\mu,{m'}}(i)} = {{\overset{\bigvee}{D}}_{\mu \quad B}^{H}{\overset{\bigvee}{C}}_{\mu \quad A}^{H}{\sum\limits_{u_{A} = 0}^{N_{c} - 1}{\sum\limits_{{u\quad B} = 0}^{N_{f} - 1}{{\overset{\bigvee}{C}}_{u\quad A}{\overset{\bigvee}{D}}_{u\quad B}{\sum\limits_{m = 0}^{M - 1}\quad {P_{u}\left\lbrack {{{\hat{H}}_{u,{m'},m}{s_{u,m}(i)}} + {{\overset{\bigvee}{D}}_{\mu \quad B}^{H}{\overset{\bigvee}{C}}_{\mu \quad A}^{H}{\eta_{m'}(i)}}} \right.}}}}}}} & (42)\end{matrix}$

[0083] With the substitution {acute over (η)}_(m′)(i):=

_(uB) ^(H)

_(μA) ^(H)η_(m), (i) equation (42) can be rewritten as equation (43)which shows the specified user, μ, is separated from MUI via multi-stageblock-de-spreading

_(μB) ^(H)

_(μA) ^(H).

y _(μm′)(i)=N _(f) P _(u) Ĥ _(μm′,m) s _(μ,m)(i)+{acute over(η)}_(m′)(i)  (43)

[0084] Multi-stage block-de-spreading output can be written in equation(44) using MK×1 blocks s_(μ)(i):=[s_(μ,0) ^(T)(i), s_(μ,1) ^(T)(i), . .. , s_(μ,M−1) ^(T)(i)]^(T). Multi-stage block-de-spreading outputy_(μ,m′)(i) and noise {acute over (θ)}(i) are M(K+L)×1 vectors generatedby concatenating the output blocks from the receiver filters matched tothe M waveforms and Ĥ_(μ) is the M(K+L)×MK matrix given by equation(45).

y _(u)(i)=N _(f) P _(u) Ĥ _(μ) s _(μ)(i)+{acute over (θ)}(i)  (44)$\begin{matrix}{{\overset{\bigvee}{H}}_{\mu}:=\begin{bmatrix}{\overset{\bigvee}{H}}_{\mu,0,0} & {\overset{\bigvee}{H}}_{\mu,0,1} & \cdots & {\overset{\bigvee}{H}}_{\mu,0,{M - 1}} \\{\overset{\bigvee}{H}}_{\mu,1,0} & {\overset{\bigvee}{H}}_{\mu,1,1} & \cdots & {\overset{\bigvee}{H}}_{\mu,1,{M - 1}} \\\vdots & \vdots & ⋰ & \vdots \\{\overset{\bigvee}{H}}_{\mu,{M - 1},0} & {\overset{\bigvee}{H}}_{\mu,{M - 1},1} & \cdots & {\overset{\bigvee}{H}}_{\mu,{M - 1},{M - 1}}\end{bmatrix}} & (45)\end{matrix}$

[0085] MS-BS accomplished by spreading matrix D_(uB), which is formed bysymbol-spreading followed by frame-interleaving, and spreading matrixC_(uA), which is formed by symbol-spreading followed bychip-interleaving and zero padding convert the conventional multi-userdetection problem into an equivalent set of single user equalizationproblems. Similar to multi-path channels with CIBS transmissions, MS-BScause ISI with each symbol block, but does not cause ICI within the codevector, i.e. ICI is replaced by ISI. Zero padding in the secondblock-spreading stage eliminates ICI and maintains the orthogonalityamong the MU and TH spreading stages.

[0086] Further, the mutli-user separating front end of receiver 14, i.e.TH and MU de-spreading units 42 and 44 respectively, preserve themaximum-likelihood (ML) optimality. This is shown through equations(46-48) given below.

:=(I _(M)

_(uA))(I _(M)

_(uB)) where

:=[

₀

,

₁, . . . ,

_(N) _(f) _(−1])  (46)

^(H)

=N _(f) I _(M(K+L))  (47) $\begin{matrix}{{\Pr\left\lbrack {x(i)} \middle| \left\{ {s_{u}(i)} \right\}_{u = 0}^{N_{u} - 1} \right\rbrack} = {\prod\limits_{u = 0}^{N_{u} - 1}\quad {\Pr \left\lbrack {y_{u}(i)} \middle| {s_{u}(i)} \right\rbrack}}} & (48)\end{matrix}$

[0087] Equation (47) implies that if η(i) is white, then η′(i) remainswhite and the probability of receiving x(i), where x(i):=[x₀ ^(T)(i), x₁^(T)(i), . . . , x_(M−1) ^(T)(i)]^(T), given that s_(u)(i) istransmitted for each user equals the product of the probabilities ofgenerating y_(u)(i) given that that s_(u)(i) is transmitted. In otherwords, de-spreading matrices

_(uA) and

_(uB) reduce a multi-user detection problem, which operates on MUI andISI in the presence of frequency selective channels, to an equivalentset of single user equalization problems without loss of ML optimality.As a result, if the N_(u) single user ISI problems in equation (44) aredemodulated in the ML sense using for example, Viterbi's decoding, thencomputationally demanding multi-user detection is not needed whenspreading and despreading matrices, D_(uB), C_(uA) and

_(uA),

_(uB), are chosen according to equations (10, 15, 29, 30) respectively.

[0088] Single-user detector 46 removes channel effects from outputblocks 45 and generates symbol block estimates given in equation (44).For example, linear equalizer Γ_(μ) outputs symbol block estimates inequation (44) according to equation (49).

ŝ _(μ)(i)=Γ_(μ) y _(μ)(i) where ŝ _(μ)(i):=[ŝ^(T) _(μ,0)(i), ŝ ^(T)_(μ,1)(i), . . . , ŝ ^(T) _(μ,M−1)(i)]^(T)  (49)

[0089] Single-user detector 46 performs symbol detection on softestimates given by equation (49) and outputs the μ^(th) user's K×1symbol block estimate 49 given by equation (50) according to decisionsmade by equation (51).

{haeck over (I)} _(μ) :=[{haeck over (I)} _(μ)(iK), {haeck over(I)}_(μ)(iK+1), . . . , {haeck over (I)}_(μ)(iK+K−1)]^(T)  (50)$\begin{matrix}{{{{{\overset{\bigvee}{I}}_{\mu}\left( {{i\quad K} + k} \right)} = {\arg \quad {\max\limits_{m}\left\{ {{\hat{s}}_{\mu,m}\left( {{i\quad K} + k} \right)} \right\}}}},{{{for}\quad {all}\quad m} \in \left\lbrack {0,{M - 1}} \right\rbrack}}{{{and}\quad k} \in \left\lbrack {0,{K - 1}} \right\rbrack}} & (51)\end{matrix}$

[0090] Trading off performance for complexity, any other detector mayreplace the maximum likelihood (ML) Viterbi equalizer, given in equation(44), typically used in multiple user detection. The complexity of MLsequence estimation (MLSE) for M-ary PPM modulation is O(KM^(L) ^(_(u))) per symbol block of size K. Consequently, the per-symbol decodingcomplexity is O(M^(L) ^(_(u)) ) regardless of the block size K and themaximum channel order. Depending on how Γ_(μ) is selected, the linearreceivers including a matched filter (MF) receiver, zero-forcing (ZF)receiver, and minimum mean-square error (MMSE) receiver are given inequations (52-54) respectively.

Γ_(μ) ^(MF) :=Ĥ _(μ) ^(H) /N _(f)  (52)

γ_(μ) ^(ZF):=(Ĥ _(μ) ^(H) Ĥ _(μ))⁻¹ Ĥ _(μ) ^(H) /N _(f)  (53)

Γ_(μ) ^(MMSE) :=N _(f) R _(μ) Ĥ _(μ) ^(H) [R _(η′)+(N _(f))² Ĥ _(μ) Ĥ_(μ) ^(H)]⁻¹  (54)

[0091] Equation 54 can be expanded using substitutions according toequations (55, 56).

R _(μ) :=E{s _(μ)(i)s ^(H) _(μ)(i)}  (55)

R _({acute over (η)}) :=E{{acute over (η)}( i){acute over(η)}^(H)(i)}  (56)

[0092] In this manner, the techniques allow transceivers to beconfigured with receivers in favor of low complexity or highperformance.

[0093]FIG. 4 illustrates an example N_(c)×N_(f) matrix 50 for assigningthe address {u_(A), u_(B)} to the u^(th) user with N_(c)=4 and N_(f)=8of system 10. In order to facilitate separation of multiple users over amulti-path communication channel each user is assigned a TH address anda user-signature pattern that will herein be referred to as a(multi-user) MU address corresponding to a TH spreading code and anorthogonal spreading code respectively. Because communication system 10uses an M-ary PPM-IRMA format, identical information bearing symbols aretransmitted over N_(f) frames. Consequently, each symbol can be viewedas being encoded with a repetition code. Replacing the repetition codewith an orthogonal spreading code specified by a MU address enablesN_(u) users to be uniquely identified. The number of users that can beuniquely identified in an M-ary PPM-IRMA format communication system isgiven in equation (57).

N_(u)=N_(c)N_(f)  (57)

[0094] Matrix 50, therefore, illustrates TH and MU address assignmentsfor a fully loaded system with 32 users. Each user is assigned a THaddress vector of length P_(c)=N_(f) according to equation (58).

{dot over (C)}_(uA) :=[ c _(uA)(0), . . . , c _(uA)(N _(c)−1)]^(T) , c_(uA)(n)∈[0, P _({dot over (c)})−1]  (58)

[0095] For any two users, u and μ, {dot over (c)}_(uA)(n):≠{dot over(c)}_(μA)(n), ∀n∈[0, P_({dot over (c)})−1]. The number of TH addressvectors can at most be N_(c), i.e. the number of chips in a frame. Ifthe number of active users, N_(u), is less than N_(c) then N_(u) of theN_(c) TH address vectors can be uniquely assigned to the N_(u) users. THspreading codes are specified by a TH address, e.g. the u_(A) ^(th), anddefined according to equation (59) with c_(uA)(n) as defined in equation(6).

c_(uA) :=[c _(uA)(0), c _(uA)(1), . . . , c _(uA)(P _(c)−1)]^(T) , c_(uA)(n)∈[0,1]  (59)

[0096] These TH spreading codes are mutually orthogonal, i.e. c_(uA)^(H)c_(μA)=N_(f)δ(u_(A)−μ_(A)), ∀u_(A),μ_(A)∈[0,N_(c)−1]. Replacing therepetition code with a set of spreading codes orthogonal to the set oforthogonal TH spreading codes allows a group of users with identical THspreading codes to be uniquely identified. MU spreading codes given by aspecific MU address, e.g. the u_(B) ^(th), can be expressed as given inequation (60).

d _(uB) :=[d _(uB)(0), . . . , d _(uB)(N _(f)−1)]^(T) , d_(uB)(n)∈[1,−1]  (60)

[0097] MU spreading codes are also designed to be mutually orthogonal.Equation (60) is designed such that d_(uB)^(M)d_(μB)=N_(f)δ(u_(B)−μ_(B)), ∀_(uB),μ_(B)∈[0,N_(f)−1]. The repetitionencoding of a standard IRMA system can be achieved by settingd_(uB)=[1,1, . . . 1].

[0098] If the number of N_(u) users satisfies N_(u)≦:N_(c)N_(f), then agiven TH address is assigned to a group of └N_(u)/N_(c)┘ or┌N_(u)/N_(c)┐ users. An additional set of MU addresses are assigned tobe able to resolve users in the same group by employing a unique mappingto each of the └N_(u)/N_(c)┘ or ┌N_(u)/N_(c)┐ users in the same group.As a result, the same MU address can be assigned to several users thatbelong to different groups since groups are differentiated via their THaddress. For example, the u^(th) user may be assigned TH address and MUaddress with index given by its {u_(A), u_(B)} pair according toequations (61, 62) respectively.

u _(A) =u(mod N _(c))  (61)

u _(B) =u(mod N _(f))  (62)

[0099]FIG. 5 is a block diagram that illustrates an example M-aryPPM-IRMA signal of the u^(th) user's information bearing symbols duringthe n^(th) chip duration. The u^(th) user's transmitted symbol duringthe n^(th) chip duration is I_(u)(└n/N_(c)N_(f)┘) 61. The orthogonalM-ary PPM of the u^(th) user in FIG. 1 can be viewed as having Mparallel branches 62 with each parallel branch realizing a shiftedversion of the pulse stream. However, one branch out of the M parallelbranches can be sufficiently selected depending on the symbol value.

[0100] The information stream, and thus s_(u,m)(└n/(N_(c)N_(f))┘) 63,does not change over the duration of N_(f)N_(c) chips (N_(f)T_(f)seconds) and is spread by TH chip-rate sequence 5 as defined in equation(6) to generate chip sequence 64 in equation (7). From this chip-ratesampled model, the nonlinearly modulated PPM-IRMA format can be viewedas a linearly modulated multiple access format such as CDMA, TDMA, orFDMA. The chip-rate sampled signal 64 is pulse shaped and converted toan analog signal 68 by functions 66. The sum of the M branches of thenonlinearly modulated PPM-IRMA signal are summed together to form signal69.

[0101]FIG. 6 illustrates the multi-user block-spreading stage with MUaddress matrices D_(uB) as given in equation (10). Data stream 24 isgenerated from data stream 22 by multi-user block-spreading unit 23within transmitter 12 of FIG. 3. Data stream 22 represents the datastream on the m^(th) branch of the u^(th) user with blocks of symbols oflength K. Multi-user block-spreading unit 23 applies Q×K block-spreadingmatrix D_(uB) to obtain data stream 24 given by the Q×1 output vectorsin equation (9). Multi-user block-spreading unit 23 can be viewed asfirst symbol-spreading each of the K symbols into Q frame-rate blocksfollowed by frame-interleaving the Q frame-rate blocks.

[0102]FIG. 7 illustrates a sub-block of the TH block-spreading stagewith TH address sub-matrix C_(uA) ⁽⁰⁾ as given in equation (14). Datastream 26 is generated from data stream 24 by TH block-spreading unit 25within transmitter 12 of FIG. 3. Data stream 26 represents the singleblock to be transmitted on the m_(th) branch of the u^(th) user and isgiven by equation (16). Each block of Q frames is spread into P chipswith L zeros padded between blocks of P chips. TH spreading unit 25 canbe viewed as first symbol-spreading frame-rate blocks into P chipsfollowed by chip-interleaving and zero padding.

[0103]FIG. 8 is a flowchart illustrating an example mode of operation ofa transmitter 12 in communication system 10 in which MS-BS preserves theorthogonality of transmitted waveforms through communication channel 16thereby deterministically eliminating MUI 18 at receiver 14. Generally,transmitter 12 parses an outbound serial data stream into blocks of Ksymbols (70) and subsequently applies Q×K MU block-spreading matrixD_(uB) (71) corresponding to the specified user's assigned MU address.This spreads blocks of K symbols into Q frames (72) and interleaves theframes for the K symbols (73).

[0104] TH spreading matrix C_(uA) of size P×Q is applied (74) accordingto the specified use's assigned TH address. TH spreading matrix C_(uA)spreads blocks of Q frames into blocks of P chips (75) and interleavesthe chips for the Q frames (76). Zeros are padded between each block ofP chips (77). The chips that are generated from the same block ofsymbols are, therefore, temporally spaced and zero padded. In thismanner, each block of K symbols produces N_(f)N_(c)(K+L) interleavedchips, where L represents the number of zeros, N_(f) represents thenumber of frames over which an identical information bearing symbol isrepeated, and N_(c) represents the number of chips per frame.Transmitter 12 converts the interleaved multi-stage block-spread chipsinto a serial bit stream (78) and generates a transmission waveform forcarrying the data through communication channel 16 to receiver 14 (79).

[0105]FIG. 9 is a flowchart illustrating an example mode of operation ofa receiver 14 in communication system 10. Receiver 14 serial to parallelconverts the received chip-rate sampled sequence P×1 blocks (80). THde-spreading matrix

_(uA) of size P×N_(f)(K+L) is applied to the P×1 blocks (81) readingsubsets of N_(f)(K+L) chips, each subset having P chips that weregenerated from the same symbol, thereby de-interleaving the chips (82).The de-interleaved chips are then de-spread into blocks of frames (83)to which MU de-spreading matrix

_(uB) of size N_(f)(K+L)×(K+L) is applied (84). K+L subsets of thestored frames, each subset having N_(f)K frames that were generated fromthe same block of symbols are de-interleaved (85) and de-spread intoblocks symbols (86). Receiver 12 then applies a single user detector toeliminate channel effects and output symbols based on the soft estimates(87).

[0106]FIGS. 10 and 11 are graphs illustrating the BER performance of amatched filter (MF), RAKE, receiver of a conventional IRMA systemagainst the MS-BS IMRA technique in the presence of frequency selectivechannels. A filter matched to the desired user's TH code and the numberof users is confined to N_(u)≦N_(c) in the conventional IRMA system. Forboth systems, parameters are adjusted to avoid ISI and symbol by symbolreception is performed at the receiver.

[0107]FIG. 10 illustrates BER performance of the conventional IRMAsystem with one, two, three, and four users compared to an MS-BS IRMAsystem with the active number of users in the range of 1≦N_(u)≦32 in anuplink scenario with perfect power control, i.e. P_(u) for all users. Inparticular, FIG. 9 illustrates how the BER performance of a conventionalIRMA system degrades as the number of users increases while the BERperformance of the MS-BS IRMA system remains invariant as the number ofactive users changes in the specified range.

[0108]FIG. 11 illustrates BER performance of the conventional IRMAsystem compared to an MS-BS IRMA system in an uplink scenario with twoactive users, user 1 being the desired user, and the effectivetransmission power of user 2 twice that of user 1. In particular, FIG.10 illustrates that the imperfect power control has no effect on the BERperformance of the MS-BS IRMA system while the conventional IRMA systemencounters degradation in BER performance.

[0109]FIGS. 12 and 13 illustrate the BER performance over 1,000 channelrealizations of the MS-BS IMRA technique compared to the MUD IRMAtechnique, which is applies only to the downlink scenario. FIG. 12illustrates the MS-BS IMRA technique compared to the MUD IRMA techniquewith a zero-forcing (ZF) linear receiver. FIG. 13 illustrates the MS-BSIMRA technique compared to the MUD IRMA technique with a minimummean-square error (MMSE) linear receiver.

[0110] Various embodiments of the invention have been described. Thedescribed techniques may provide advantages in multi-user communicationsystems. Conventional IRMA systems approximate MUI as Gaussian noise anduse TH codes with strict power control to statistically suppress MUI.Applying a first and a second block-spreading code selected from a firstand a second set of mutually orthogonal block-spreading codes in whichthe first and the second set of mutually orthogonal block-spreadingcodes are also orthogonal preserves the orthogonality of transmittedwaveforms through frequency selective communications. As a result, MUIand ISI can be eliminated deterministically. Such systems allow eachuser to use a different detector that can be chosen in favor for lowcomplexity or high performance without interfering with other users.

[0111] “Multi-stage block-spreading” may use a first set of mutuallyorthogonal MU spreading codes, e.g. direct sequence CDMA codes ordigital carrier frequency multiple access codes, and a second set ofmutually orthogonal TH codes. The number of orthogonal mutuallyorthogonal MU spreading codes and TH spreading codes may be equal to thenumber of frames over which identical information bearing symbols,N_(f), are transmitted and the number of chips per frame, N_(c),respectively. As a result, a group of users may be assigned the same THcode with each user in the group uniquely identified by a different MUcode. The maximum number of uniquely identified users in a MS-BS IRMAsystem is, therefore, N_(c)N_(f), with each user identified by a MUaddress and TH address corresponding to a specific MU and TH coderespectively.

[0112] Further, such systems do not exhibit BER degradation as thenumber of users increases and requires no additional power to achieve aspecified BER regardless of the multi-path channel. Different users mayalso be provided with variable transmission rates by assigning a singleuser more than one set of MU and TH addresses.

[0113] The described techniques can be embodied in a variety of devicesthat communicate using ultra wideband communication, including basestations, mobile phones, laptop computers, handheld computing devices,personal digital assistants (PDA's), a device within a personal areanetwork, a device within a sensor network, and the like. The devices mayinclude a digital signal processor (DSP), field programmable gate array(FPGA), application specific integrated circuit (ASIC) or similarhardware, firmware and/or software for implementing the techniques. Ifimplemented in software, a computer-readable medium may store computerreadable instructions, i.e., program code, that can be executed by aprocessor or DSP to carry out one of more of the techniques describedabove. For example, the computer-readable medium may comprise randomaccess memory (RAM), read-only memory (ROM), non-volatile random accessmemory (NVRAM), electrically erasable programmable read-only memory(EEPROM), flash memory, or the like. The computer-readable medium maycomprise computer readable instructions that when executed in a wirelesscommunication device, cause the wireless communication device to carryout one or more of the techniques described herein. These and otherembodiments are within the scope of the following claims.

1. A method comprising: generating a stream of frames from blocks ofinformation bearing symbols, wherein the frames corresponding todifferent blocks of the symbols are interleaved; generating a stream ofchips from the stream of frames, wherein the chips corresponding todifferent frames are interleaved; and outputting an ultra wideband (UWB)transmission signal from the stream of chips.
 2. The method of claim 1,wherein generating a stream of frames comprises: parsing the symbolsinto blocks of K symbols; applying an orthogonal set of spreading codesto the blocks of K symbols to form Q frames; and interleaving the Qframes to form the stream of frames.
 3. The method of claim 2, whereinapplying an orthogonal set of spreading codes comprises applying directsequence code-division multiple access codes or digital carrierfrequency division multiple access codes.
 4. The method of claim 2,wherein generating a stream of chips comprises: applying an orthogonalset of time-hopping spreading codes to the interleaved frames togenerate a plurality of chips for each frame; and interleaving each ofthe plurality of chips to form the output stream of chips.
 5. The methodof claim 4, wherein generating a stream of chips further comprises:storing the chips in an array having M columns and K+L rows, where L isa function of the communication channel length; and padding each columnof the array with L guard chips.
 6. The method of claim 5, wherein theguard chips comprise null values.
 7. The method of claim 5, whereinoutputting the transmission signal by reading the chips from the arrayin column-wise fashion.
 8. The method of claim 5, wherein the set ofspreading codes and the set of time-hopping spreading codes are mutuallyorthogonal so that the interleaved and padded chips retain theirorthogonality after passing through a multi-path communication channel.9. The method of claim 5, further comprising: assigning each of the setof spreading codes to a different user of a group of users; andassigning each user of the group a common one of the set of time-hoppingspreading codes.
 10. The method of claim 5, further comprising assigningunique addresses to users as unique pair-wise combinations of the set oforthogonal spreading codes and the set of time hopping spreading codes.11. The method of claim 10, wherein the total number of users N_(u)supported by the wireless communication devices equals N_(c)*N_(f),where each of the information bearing symbols is repeated over N_(f)frames and each frame includes N_(c) chips. 12 The method of claim 11,wherein each of the unique addresses comprises a unique multiple useraddress (u_(B)) selected from set of spreading codes in combination witha unique time-hopping address (u_(A)) selected from the set oftime-hopping codes.
 13. The method of claim 12, further comprisingselecting the unique multiple addresses and the unique time-hoppingaddresses in accordance with: u _(A) =u(mod N _(C)), and u _(B) =└u/N_(c)┘.
 14. The method of claim 1, further comprising: receiving thesignal; and outputting a stream of estimate symbols from the signalusing a two-stage de-spreading unit having a time-hopping de-spreadingmodule and a multi-user de-spreading module.
 15. The method of claim 14,wherein outputting a stream of estimate symbols comprise: converting thesignal to a stream of chips; applying a first de-spreading matrix withthe time-hopping de-spreading module to de-interleave the chips intoblocks of frames; applying a second de-spreading matrix to the blocks offrames with the multi-user de-spreading module to de-interleave theframes and produce blocks of estimate symbols; and applying a singleuser detection scheme to the blocks of estimate symbols to output thestream of the estimate symbols.
 16. The method of claim 15, whereinapplying first and second de-spreading matrices deterministicallyeliminates multiple user interference.
 17. The method of claim 15,wherein applying a first de-spreading matrix comprises: parsing thechips into blocks of P chips; and applying a time-hopping de-spreadingmatrix of size P×N_(f)(K+L) to the blocks, where each of the informationbearing symbols is repeated over N_(f) frames, L is a function of thecommunication channel length, and the stream of frames was generatedduring transmission using blocks of K symbols.
 18. The method of claim15, wherein applying a second de-spreading matrix comprises applying asecond de-spreading matrix of size N_(f)(K+L)×(K+L) matrix, where eachof the information bearing symbols is repeated over N_(f) frames, L is afunction of the communication channel length, and the stream of frameswas generated during transmission using blocks of K symbols.
 19. Awireless communication device comprising: a multiple-userblock-spreading unit that generates a set of frames for respectiveblocks of information bearing symbols and produces a stream of frames inwhich the frames from different sets are interleaved; a time-hoppingblock-spreading unit that generates a set of chips for each frame andoutputs a stream of chips in which the chips generated from differentframes are interleaved; and a pulse shaping unit to output an ultrawideband (UWB) transmission signal from the stream of interleaved chips.20. The wireless communication device of claim 19, wherein themultiple-user block-spreading unit produces the stream of frames byparsing the symbols into blocks of K symbols, applying an orthogonal setof spreading codes to the blocks of K symbols to form Q frames andinterleaving the Q frames to produce the stream of frames.
 21. Thewireless communication device of claim 20, wherein the set of spreadingcodes comprises direct sequence code-division multiple access codes. 22.The wireless communication device of claim 20, wherein the time-hoppingblock-spreading unit generates the stream of chips by applying anorthogonal set of time-hopping spreading codes to the interleaved framesto generate a plurality of the chips for each frame, and interleavingeach of the plurality of chips to form the output stream of chips. 23.The wireless communication device of claim 22, wherein the time-hoppingblock spreading unit comprises memory to store the chips in an arrayhaving columns and rows, where the number of rows in the array is afunction of the communication channel length, and wherein thetime-hopping block spreading unit pads each column of the array withguard chips, and outputs the transmission signal by reading the chipsfrom the array in column-wise fashion.
 24. The wireless communicationdevice of claim 22, wherein the set of spreading codes and the set oftime-hopping spreading codes are mutually orthogonal so that theinterleaved and padded chips retain their orthogonality after passingthrough a multi-path communication channel.
 25. The wirelesscommunication device of claim 24, wherein the wireless communicationdevice stores a unique address assigned to one of a plurality of users,and the unique address is formed from a pair-wise combination of one ofthe set of orthogonal spreading codes and one of the set of time hoppingspreading codes.
 26. The wireless communication device of claim 19,wherein the wireless communication device comprises one of a basestation and a mobile device, a device within a personal area network, ora device within a sensor network.
 27. A wireless communication devicecomprising a two-stage despreading unit that processes a received ultrawideband (UWB) transmission signal to produce estimate symbols, whereinthe received UWB signal comprises a multi-user block-spread UWB signalformed from interleaved symbol frames and interleaved chips within thesymbol frames.
 28. The wireless communication device of claim 27,wherein the two-stage de-spreading unit comprise: a time-hoppingde-spreading module that applies a first de-spreading matrix tode-interleave the chips into blocks of frames, and a multi-userde-spreading module that applies a second de-spreading matrix tode-interleave the frames and produce blocks of the estimate symbols. 29.The wireless communication device of claim 28, wherein the wirelesscommunication device comprises one of a base station and a mobiledevice.
 30. A system comprising: a wireless transmitter to transmit anultra wideband (UWB) signal according to interleaved chips generatedfrom interleaved frames produced by blocks of information bearingsymbols; and a wireless receiver to receive the UWB signal andde-interleave the chips and frames to produce estimate symbols.
 31. Thesystem of claim 30, wherein the transmitter comprises: a multiple-userblock-spreading unit that generates a set of the frames for therespective blocks of information bearing symbols and produces a streamof the frames in which the frames from different sets are interleaved; atime-hopping block-spreading unit that generates a set of the chips foreach of the frames and outputs a stream of the chips in which the chipsgenerated from different frames are interleaved; and a pulse shapingunit to output the UWB transmission signal from the stream ofinterleaved chips.
 32. The system of claim 30, wherein the receivercomprises: a time-hopping de-spreading module that applies a firstde-spreading matrix to the UWB signal to de-interleave chips into blocksof frames, and a multi-user de-spreading module that applies a secondde-spreading matrix to de-interleave the frames and produce blocks ofthe estimate symbols.
 33. A computer-readable medium comprisinginstructions to cause a programmable processor of a wirelesscommunication device to: generate a stream of frames from blocks ofinformation bearing symbols, wherein the frames corresponding todifferent blocks of the symbols are interleaved; generate a stream ofchips from the stream of frames, wherein the chips corresponding todifferent frames are interleaved; and output an ultra wideband (UWB)transmission signal from the stream of chips.